The earthquake in Haiti has left an estimated 1.5 million people homeless and tens of thousands without access to food, water and medical supplies. The UN says the scale of the disaster is "historic", with its staff confronting devastation and logistical problems on a scale never seen before.
Here is a look at some of the issues agencies say have hampered the aid effort and how they are being dealt with.
SCALE OF DISASTER
The devastation has been described as 'historic' in scale
Aid agencies say Haiti is quite simply one of the worst disasters they have ever handled.
"In every direction the task is huge, it is an historic challenge" UN spokesman Elisabeth Byrs told the BBC.
"We are trying to run an operation for three million people, the task is huge and the coordination is immense."
They say that while images of the misery in Haiti can spread rapidly around the world, aid supplies and skilled people cannot travel so fast.
"Haiti is definitely the most complex emergency to date," said Jean Philippe Chauzy of the International Organization for Migration (IOM).
He said it was difficult for those not involved in emergency aid to understand how difficult the operations can be.
"The fact that the reporting is immediate might give the impression that aid can be sorted out in such a fashion, but you can't do that because of the scale of the operation and complexity."
AIRPORTS AND PORTS
Aid groups say supplies have been stuck at the airport
While the main airport in Port-au-Prince was not put out of action by the quake, it is not equipped to deal with the volume of flights arriving.
There have been complaints of huge backlogs, with some aircraft circling for hours or being diverted to the Dominican Republic. Paul Peachy of Christian Aid said it had difficult even getting emergency staff to Haiti.
The UN says 150 planes are now landing daily in Port-au-Prince but the US Army, which has taken over control of the airport, says 1,500 planes are still scheduled to arrive.
To speed up the process, aid flights are also coming into and out of the neighbouring Dominican Republic and in smaller airports in Haiti.
Port-au-Prince's main port was also badly damaged by the quake and other ports in the area could only accept smaller vessels. One of the two piers in the capital's docks reopened on Thursday, allowing bulkier shipments to be delivered by sea.
But on Thursday, Haiti's President Rene Preval joined several aid agencies in saying it is what happens when aid arrives in the country that is the problem.
"People say to us: Where are the trucks to transport the aid? Where are the depots to store what arrives?" said Mr Preval.
"What's important is coordination of the aid, so that we know what we receive, in what quantity, when and how it's distributed."
DAMAGE TO ROADS
Damaged roads make it harder to move resources around the country
Moving aid to where it is needed has been hampered by damage to roads, piles of rubble in the streets or by the sheer volume of people trying to move out of or around the capital.
"Road corridors are heavy with traffic, so travelling takes hours," said Ms Byrs of the UN.
Aid agencies also report a lack of trucks and a lack of fuel but said supplies were now starting to arrive. They also hope to start employing local people soon in rubble clearing so reconstruction can begin.
The fact that many roads were damaged also meant that survivors have tended to gather in small groups, rather than in larger camps, he said.
"It is very difficult to reach out systematically when you've got hundreds of groups of people in Port-au-Prince and the vicinity," he said.
"If you manage to get tens of thousands of displaced people in one place you obviously streamline the distribution."
The US has carried out air drops of basic supplies in some areas, but this is considered a last resort as it is considered inefficient and can lead to unrest on the ground as people compete for the few supplies.
Aid groups were unable to broadcast information to camps
Phones and internet links were down in many areas of Haiti for some time after the quake, making it harder for agencies to coordinate their efforts.
Mr Peachy of Christian Aid said it had been impossible to contact his colleagues in Haiti for the first 48 hours after the quake.
But Mr Chauzy of the IOM said the lack of communications also meant it was hard to tell people needing aid where they could go to collect it.
"Now the media is up and running, local radio station have started broadcasting and the UN's Radio Minustah has started broadcasting in Creole," he said.
Many NGOs based in Haiti lost staff and equipment in the disaster, which meant the systems they had in place could not be used. Christian Aid was one of many to lose its entire building.
Aside from the practical losses, many aid workers are also dealing with grief as they carry out their work.
"For the moment we are concentrating - and I would say escaping - with the work. After, we will experience the emotions," said Ms Byrs at the UN.
Armed police have been deployed to deter looting and violence
There has been concern about the security situation in Haiti, with fears that people not receiving aid would turn to violence.
John O'Shea of Irish charity Goal told the Guardian newspaper he could not allow aid workers to move into Haiti from the Dominican Republican because he had "no guarantee that the people driving them are not going to be macheted to death on the way down".
But while there have been reports of looting and some incidents of violence, other agencies say they have been impressed by how Haitians have responded to the disaster.
Ms Byrs said the capital was "tense but calm" and that the few examples of violence were not representative of Haitian people.
"We are seeing a huge solidarity between Haitian people. They resilient, they taken their fate into own hands from the very beginning," she said.
Jean Philippe Chauzy of French agency Acted said the security issues in Haiti should neither be over-played nor used as an excuse to prevent aid deliveries.
"The needs are there, people are desperate, if you don't distribute the assistance, people will be even more desperate and ready to resort to anything."
On Thursday, a truck run by the US-based Catholic Relief Service was reported to have been overrun by desperate people when it arrived at a makeshift camp in the town of Leogane.
But Adrien Tomarchio of Acted says the main safety concern has been for those people receiving aid.
"The best process is not to start distribution at once and announce it so everyone comes," he says.
"We make sure we can set up a proper secured distribution point, where people can come one-by-one. The aim is to deliver to the most vulnerable people first, then we also can focus on other groups."
The US Army has been deployed in vast numbers in Haiti, both to help with the aid effort and to help maintain law and order.
The US has tens of thousands of troops in the country
Medecins Sans Frontieres (MSF) has complained that the rush to get troops into the country has been at the expense of the delivery of humanitarian supplies.
"Everything has been mixed together and the urgent and vital attention to the people have been delayed while military logistics - which is useful but not on day three, not on day four, but maybe on day eight - has really jammed the airport and led to this mismanagement."
MSF say one of its planes carrying 12 tonnes of medical supplies was repeatedly turned away from the airport despite having prior permission to land.
John O'Shea of Goal said the failure of the UN and US to work together was leading to "a situation of utter chaos".
The UN has dismissed such criticism, saying it "underestimated the logistical difficulties" and that the US was the only country in the region capable of providing logistical support on the scale needed.
"We are not talking about politics, this is humanitarian. Our goal is to delivery assistance as soon as possible and co-ordination is vital - without it you can't get the right aid to the most vulnerable," said spokeswoman Ms Byrs.
Aid agencies are keen to stress that the response to a disaster such as Haiti must be responsible and durable. The last thing they want is for the mechanisms they put in place to lead to long term harm for the people they are trying to help.
Badly managed distributions can mean the most needy go without
Adrien Tomarchio, of French agency Acted, told the BBC the aim when distributing food is to get it to the right people quickly, rather than just get it out quickly.
"If we distribute food all at once, some people will take more then they need and there is the risk of them selling food items, rather than it reaching the people that need it," he said.
"The aim is to deliver to the most vulnerable people, then we can focus on other groups."
In the case of shelter, there is little point in building a camp for displaced people without confirming they will be able to stay there, possibly for many months.
Agencies have had to work with the local authorities to determine whether the land is suitable, whether it can be properly equipped with shelter and sanitation.
Land rights issues also do not disappear after a disaster, so agencies have to establish who owns the land on which they hope to build.
"You can't just go there and get land, level it and start building," says Jean Philippe Chauzy of IOM.
Haiti's rainy season begins in May or June, so camps cannot be placed in areas which are likely to be flooded in the next few months.
IOM, working with troops from Minustah, have started clearing a patch of land in a suburb about 10km out of Port-au-Prince, but say further camps will almost certainly be needed.
As recovery begins, it is important that as many Haitians that can return to work do. A camp which is too far away from the capital for them to be able to travel in for work will benefit no one.